1. Statement of the Technical Field
The present invention relates to radio frequency identification (“RFID”) systems and more particularly to a system and method for detecting the presence and absence of items in a given RFID interrogation zone.
2. Description of The Related Art
Radio frequency identification (“RFID”) is a term used to describe technologies that use radio waves to automatically identify objects or people. RFID systems are used for many applications such as managing inventory, electronic access control, security systems, automatic identification of cars on toll roads, article surveillance, etc. RFID implementations are accomplished in several ways. The most popular involves storing a serial number that identifies an object or a person, and perhaps other information, on a microchip in communication with a transponder. This arrangement is commonly referred to as an RFID marker or RFID tag. RFID systems may be used to track or monitor the location and/or status of articles or items to which an RFID marker is applied.
An antenna, sometimes packaged with a transceiver and decoder, provides the ability to receive and convert the radio waves reflected back from the RFID marker into digital information that can then be passed on to computers for processing. The antenna, the transceiver and the decoder are often collectively referred to as the RFID reader. The RFID reader enables transmission of an interrogation signal to the RFID marker to obtain identification information. Based on the interrogation signal, the RFID reader also receives signals from the marker bearing the identification information. The region in which an RFID reader can detect the presence of RFID markers is referred to herein as an “interrogation zone”.
Signals received by RFID readers in the form of backscatter modulation are typically analyzed in order to obtain marker identification information from the interrogated markers. While this leads to readers that are designed to be sensitive to reflected energy, normally, extraneous reflections unrelated to the identity of the marker, are removed or mitigated in some way by the receiver portion of the RFID reader.
In some RFID systems, items which are to be monitored are stacked near each other or one on top of another, on shelves or other limited-space locations. Often, there may be a need to determine when some or all of the items on the shelf have been removed. For example, in a music store, a shelf might contain many compact discs. As customers remove discs and take them to the cashier for purchase, the supply will run low and management should be made aware of this so the purchased discs can be replaced on the shelf. Or, in a warehouse, boxes containing items may be stored on shelves. Again, as these boxes are taken off the shelves, it is important to note this since new replacement items may have to be ordered.
In close-quarter interrogation zones such as on a shelf, items may be in close proximity to the RFID reader antenna. It is sometimes necessary to use a higher power than normal to transmit and receive through all items in a stack in order to read items at, for example, the top of the stack. However, because there are often multiple RFID interrogation systems situated near each other, if it is determined that items have been removed from a monitored zone, the RF energy in that zone is now free to propagate throughout the zone and to surrounding areas causing interference with other nearby RFID interrogation systems.
Therefore, what is needed is a method and system that can be used to accurately determine when items within an RFID interrogation zone are being depleted and to take corrective action such as initiating processes to replenish the removed items and/or reduce the power of the RFID readers to conserve costs and to reduce the likelihood of interference with other RFID readers.